Field of the Invention
The invention relates to a method for determining a temperature at a spot weld between two parts of a first metal to be welded to one another, the joint being produced with an electrode of a second metal which is placed in contact with at least one of the parts to be welded. The invention further relates to a method and an apparatus for performing the method in examining the quality of the spot-welded joint.
Spot welds are in widespread use, for example in automobile construction, where metallic parts, preferably sheet-metal parts, are intended to be joined to one another. Preferably, when the spot-welded joint is not only intended to be used for a temporary bonding together of the parts but represents the final joint between the parts, it must be ensured that the joint can withstand the operating loads. The determining factor of the quality of such a spot weld is adequate heat development and an adequate molten pool size under the electrodes. For example, if the welding current flows through shunt paths, that can lead to so-called dry joints having inadequate strengths. The requirement exists, primarily in the course of automated manufacturing processes, for an examination of the quality of such spot welds, which can be carried out at a lower cost in terms of time and equipment than the methods which are intrinsically possible and are otherwise used for material testing (for example by means of X-rays or ultrasound). It is known for sample inspections to be carried out by forcibly tearing apart such welded joints.
An article entitled "Experimental Measurement of Liquid Nugget Heat Convection in Spot Welding" by W. V. Alkini, in Welding Journal, vol. 69, 1990; No. 5; Miami, Fla., describes the temperature distribution within a molten nugget at an AC spot weld. The measurement is performed with a plurality of micro-thermocouples with a very short response time disposed at the spot weld location. A temperature curve at the spot weld is determined with each micro-thermocouple. The use of thermocouples at the spot weld is not suitable in determining the temperature at a plurality of spot welds, for instance during an automatic manufacturing process, however, particularly because of excessive demands in terms of equipment and measurement technology as well as because of time constraints.
The German Gebrauchsmuster DE-GM 72 23 324 describes a device for monitoring electrical resistance welds with a welding apparatus which provides for at least one electrode contacting the material to be welded. The device includes a heat radiation sensor which is directed at a spot weld and which has an evaluation unit connected downstream thereof. The heat radiated from the spot weld during the welding operation is received by the heat radiation sensor and from that there is derived a control signal, in particular the temperature at the spot weld. The temperature at the spot weld is exclusively determined during the welding operation and it is compared with a minimum temperature and a maximum temperature. When the temperature lies between the minimum and the maximum temperatures, then the welded joint produced by the welding operation is characterized as satisfactory. The heat radiated after the welding operation is measured and evaluated only for the purpose of preparing the evaluation device for a new measurement.
U.S. Pat. No. 3,800,119 to Kuroda et al. describes a device for monitoring the quality of a spot weld between two steel plates to be welded to one another, whereby the spot weld is produced by means of two mutually oppositely disposed welding electrodes. In that device, only information obtained during the welding process is used for determining its quality. Such determination is based on the high welding voltage measured between the welding electrodes during the welding process. The difference between the weighted maximum value of the welding voltage and the value of the welding voltage at the end of the welding process serves as the basis for determining the quality.
The Soviet inventor's certificate SU 1558-609 A describes a spot weld monitoring device. The device includes two adjacently disposed welding electrodes which are in contact with one of two parts to be welded to one another. After the welding operation, each of the electrodes form a thermocouple with that object. The two measured thermal voltages are compared. If the thermal voltages correspond to one another, a spatially constant temperature distribution at the spot weld is presumed. Accordingly, the device only serves to monitor the spatial constancy of the temperature at a spot weld, without knowing the actual temperature, or the actual temperature difference.